Llys Rhosyr was a stronghold of Welsh rule in the 13th century and it’s still possible to see the layout of this court of the Princes of Gwynedd, as the ground plan is virtually intact.

The best way to appreciate Rhosyr is to first see the audiovisual presentation at the Prichard Jones Institute in nearby Newborough before visiting the site, 900 yards away. The Princes governed their realm through a local administration based on commotes, essentially small counties sub-divided into smaller townwships. They controlled these by developing a peripatetic court, staying in each commote on an irregular cycle rather than establishing a singular capital per-se. Each commote maintained a suite of buildings in readiness for the visit of the princely entourage; these were called Llysoedd and were pivotal to the cohesiveness of Gwynedd. Each Llys was the centre of a small Royal estate, or Maerdref that provided directly for the Princes or their officials, who presided in the Princes' absence. Llys Rhosyr was the heart of Menai commote and is the most complete site to have been positively identified and partially excavated. Since 1992 over a quarter of the walled site has been exposed, yielding a number of finds, notably pottery, coins and other small artefacts. Rhosyr was probably largely abandoned in the years after the conquest of 1282. In the winter of 1330 a storm buried the site beneath sand dunes. Archaeologists have so far exposed the enclosure wall and three main buildings of the Llys. These are a large main hall and an adjacent building thought to have housed the Prince's private rooms.

Llys Rhosyr is a significant site not least because it is the only Royal Court of the Princes of Gwynedd of which the ground plan remains virtually intact. Those elsewhere were despoiled at the conquest, or destroyed in the last century.